1. Field of the Invention
The present invention relates to an acceleration sensor for detecting acceleration, which is used for toys, automobiles, aircrafts, portable terminals and the like, and particularly to an acceleration sensor that can be produced using a semiconductor technology.
2. Description of the Related Art
Acceleration sensors utilizing a change in physical quantity such as a piezo resistance effect and a change in electrostatic capacity have been developed and commercialized. These acceleration sensors can be widely used in various fields, but recently, such small-sized acceleration sensors as can detect the acceleration in multi-axial directions at one time with high sensitivity are demanded.
Since silicon single crystal becomes an ideal elastic body due to the extreme paucity of lattice defect and since a semiconductor process technology can be applied for it without large modification, much attention is paid to a piezo resistance effect type semiconductor acceleration sensor in which a thin elastic support portion is provided at a silicon single crystal substrate, and the stress applied to the thin elastic support portion is converted into an electric signal by a strain gauge, for example, a piezo resistance effect element, to be an output.
As a conventional triaxial acceleration sensor, there is the one disclosed in, for example, Japanese Laid-Open Patent No. 63-169078, and its plan view is shown in FIG. 11, and a sectional view taken along the XIIxe2x80x94XII line in FIG. 11 is shown in FIG. 12, and a perspective view is shown in FIG. 13. The acceleration sensor 200 has elastic support arms 230 each of a beam structure, constituted by a thin portion of a silicon single crystal substrate. A mass portion 220 in a center, which is constituted by a thick portion of a silicon single crystal substrate, and a frame 210 in a periphery thereof are connected by the elastic support arms 230. A plurality of strain gauges 240 are formed in each axial direction on the elastic support arms 230.
An entire structure will be explained, referring to FIG. 11, FIG. 12 and FIG. 13. The sensor 200 has the mass portion 220 constituted by the thick portion of the silicon single crystal substrate, a frame 210 placed to surround the mass portion 220, and two pairs of elastic support arms 230 in a beam form, which are perpendicular to each other and each constituted by the thin portion of the silicon single crystal substrate to bridge the mass portion 220 and the frame 210. When the acceleration works, the mass portion moves in the frame to deform the elastic support arms, and thus the deformation is detected by the strain gauges provided on the elastic support arms to obtain the acceleration that works. The acceleration in an X-axis direction in FIG. 11 is measured by the four strain gauges 240 provided on the elastic support arms extending in the X-axis direction, and the acceleration in a Y-axis direction is measured by the four strain gauges 240 provided on the elastic support arms extending in the Y-axis direction. The acceleration in a Z-axis direction is measured by means of all the strain gauges 240. By making four L-shaped through-holes 250 in the silicon single crystal substrate having the size of the frame 210, the mass portion 220 in the center, the frame 210 in the periphery and the support arms 230 bridging them are formed, and by making the support arm portions thin, the acceleration sensor is constructed to be deformable and highly sensitive.
In the above-described acceleration sensor, to enhance sensitivity, it is effective to increase the volume of the mass portion 220 to increase the mass, or to increase the length of the elastic support arms 230, and as is generally well-known, the sensitivity increases substantially in proportion to the mass of the mass portion and the length of the support arms. That is, the volume of the mass portion 220 is increased, or the length of the elastic support arms 230 are increased, whereby the elastic support arm 230 becomes more deformable and the stress can be effectively transmitted to the strain gauges, thus enhancing sensitivity. However, increasing the mass portion 220 and increasing the length of the elastic support arms 230 are mutually contradictory, and both of them are not mutually compatible especially under the condition that the chip size is kept constant, or when reduction in size is planned. That is, if the mass portion 220 is made larger, the length of the elastic support arms 230 becomes smaller, and a great deal of improvement in sensitivity cannot be improved. Thus, glass pieces and the like are bonded on the back surface of the mass portion 220 in assembly process to increase the volume (that is, weight) of the mass portion 220, whereby the sensitivity is enhanced. The length of the elastic support arms 230 cannot be made large, while the chip is made larger in the thickness direction (the thickness direction of the silicon single crystal substrate), whereby the mass of the mass portion 220 is increased to enhance the sensitivity. Accordingly, it is conventionally impossible to realize a compact and thin acceleration sensor with high sensitivity.
The present invention is accomplished in view of the above-described circumstances, and its object is to solve the above-described problem and provide a compact and thin acceleration sensor capable of enhancing sensitivity.
In order to solve the above-described problem, the present invention adopts an acceleration sensor as follows. That is, the acceleration sensor of the present invention comprises a mass portion provided in a center; a thick frame surrounding the mass portion with a predetermined distance from the mass portion; a plurality of elastic support arms extending from an edge of a top surface of the mass portion, bridging the top surface edge of the mass portion and an inside edge of a top surface of the thick frame and hanging the mass portion inside of the thick frame; and a plurality of strain gauges disposed on the elastic support arms. The mass portion has the top surface, a bottom surface opposite to the top surface and a plurality of side walls surrounding the mass portion between the top surface and the bottom surface of the mass portion. The thick frame has the top surface, a bottom surface opposite to the top surface and inside walls on inside surfaces, facing the mass portion, of the thick frame between the top surface and the bottom surface of the thick frame.
The acceleration sensor is made of a silicon single crystal wafer, preferably a SOI (Silicon-on-insulator) wafer, and can be constructed to have a thick-walled frame with its plane shape being substantially a square, a mass portion which is provided at a center of the thick-walled frame and is formed to be substantially a square, and four elastic support arms which connect centers of sides of the square on the top surface of the mass portion and centers of inner sides of the thick-walled frame in a square shape which is on the top surface of the thick frame. In the acceleration sensor made of a silicon single crystal wafer or a SOI wafer, the top surface of the thick-walled frame, the top surface of the mass portion and the top surfaces of the four elastic support arms are formed by using the surface of one side of the wafer, and therefore they are on substantially the same surface. A bottom surface of the thick-walled frame and a bottom surface of the mass portion are formed by using a surface of the other side of the wafer, and therefore they are on substantially the same surface. The elastic support arm is formed to be thin by being cut by etching or the like from the surface of the other side of the wafer, and therefore when it is made of the SOI wafer, it is formed of a remaining SiO2 layer, or a laminated product of the SiO2 layer and a silicon layer.
In the acceleration sensor of the present invention, at least either of the mass portion or the thick-walled frame expands in its width from the top surface toward the bottom surface, in the cross section perpendicular to its top surface. When the cross section perpendicular to the top surface of the square mass portion expands in the width from the top surface to its bottom surface, one side of the square on the bottom surface is longer than one side of the square on the top surface. When the vertical cross section of the thick-walled frame expands in its width from its top surface to its bottom surface, the width of the bottom surface is longer than the width of the top surface of the thick-walled frame. It is preferable that in the acceleration sensor of the present invention, the mass portion extends in the width from the top surface toward the bottom surface in the cross section perpendicular to its top surface, and the thick-walled frame expands in the width from its top surface toward its bottom surface in the cross section perpendicular to its top surface.
The dimension of the mass portion and/or the thick-walled frame on the top surfaces is made small and the dimension on the bottom surfaces is made large, whereby the length of the elastic support arm can be made larger without reducing the mass of the mass portion, therefore making it possible to increase sensitivity with which acceleration is detected. If the dimension of the mass portion and the thick-walled frame on their top surfaces is made small and the dimension on their bottom surfaces is made large, the elastic support arms can be extended on the side of the mass portion and on the side of the thick-walled frame. Alternatively, as for only one of the mass portion and the thick-walled frame, the dimension on the top surface can be made small and the dimension on the bottom surface can be made large. However, when the dimension of one of them is changed, it is preferable to apply the present invention to the mass portion side. If the dimension of the thick-walled frame on its top surface is made small and the dimension on its bottom surface is made large, a kind of notch is formed in a portion of the thick-walled frame at which the elastic support arm is attached. If the dimension on its top surface is made extremely smaller as compared with the dimension on its bottom surface, the notch formed at the portion of the thick-walled frame at which the elastic support arm is attached is deepened, which causes the fear of reducing mechanical strength of a base supporting the elastic support arm.
Expressing the acceleration sensor of the present invention in other words, on at least one of each side wall of the mass portion and each inner wall of the thick-walled frame, the angle, which is formed by a plane passing through the place on the wall at which the elastic support arm is attached and the intersection line of the wall and the bottom surface corresponding to the wall, and the bottom surface corresponding to the wall, is 80 degrees or larger and smaller than 90 degrees. Preferably, the angle, which is formed by the plane passing through the place on the wall, at which the elastic support arm is attached, on each side wall of the mass portion and passing the intersection line of the wall and the bottom surface corresponding to the wall, and the bottom surface corresponding to the wall, is 80 degrees or larger and smaller than 90 degrees, and the angle, which is formed by a plane passing through a place on the wall at which the elastic support arm is attached on each of the inner wall of the thick-walled frame and passing the intersection line of the wall and the bottom surface corresponding to the wall, and the bottom surface corresponding to the wall, is 80 degrees or larger and smaller than 90 degrees.
Further expressing the acceleration sensor of the present invention in other words, the angle, which is formed by at least one of each side wall of the mass portion and each inner wall of the thick-walled frame, and the bottom surface corresponding to the wall, is 80 degrees or larger, and smaller than 90 degrees. Preferably, the angle, which is formed by each side wall of the mass portion, and the bottom surface corresponding to the wall, is 80 degrees or larger and smaller than 90 degrees, and the angle, which is formed by each of the inner walls of the thick-walled frame and the bottom surface corresponding to the wall, is 80 degrees or larger and smaller than 90 degrees.